1. Field of the Invention
This invention relates generally to a fiber-optic sensor for measuring temperature. More specifically, this invention relates to a fiber-optic temperature sensor constructed of optical fiber into which permanent microbends have been introduced.
2. Description of the Prior Art
In certain situations, fiber optic temperature sensors may be more suitable than their electronic equivalents because of their immunity to electromagnetic interference and their inability to create potentially hazardous sparks. Because of these qualities, fiberoptic sensor research has significantly expanded during the past decade. Fiber optics has matured in the application of fiber optics as sources, detectors, and other optical components.
It is well-established that small random or periodic undulations in the direction of an optical fiber's axis, known as microbends, can cause a significant reduction in the fiber's optical transmission. Since this phenomenon hinders optical transmission, most of the prior art is concerned with minimizing this effect.
Wlodarczyk et. al., U.S. Pat. No. 4,678,903, teaches a method for the self-alignment of a microbend sensor contained within a tubular housing which is configured with internal threads and an inner expandable insert with external threads to induce microbends in an optical fiber. Expansion of an inner tube against an outer tube, under internal pressure, produces the non-existent microbends.
Falco et. al., U.S. Pat. No. 4,846,547, relates to a fiber-optic detector capable of producing microbends in an optical fiber in response to the variation of a physical quantity or of an environmental parameter such as temperature. Falco describes a confining structure comprising a two-component sheath whose components contain deformations and expand at different rates resulting in periodic deformations which are expected to be impressed on an optical fiber. To produce such a sheath requires the manufacture of non-existent periodic deformations in an optical fiber using photo-etching techniques and a two-component sheath.
Franke et. al., U.S. Pat. No. 4,039,248, describes a method of cabling an optical fiber so as to minimize the influence of temperature and other external perturbations on the fiber's transmission. Franke et. al. does not describe the use of an optical fiber as a temperature sensor. Franke configures the optical fiber within a sheath in a sinusoidal fashion through the use of studs or clamps to secure its position and avoid microbends in the optical fiber. Van der Hock, U.S. Pat. No. 4,468,088, casts doubt upon the desired function described in Franke (U.S. Pat. No. 4,039,248) as Van der Hock notes the possibility of the fiber developing micro cracks and finally breaking.
U.S. Pat. No. 5,132,529 (Weiss, the present inventor) measures changes in a fiber's optical transmission when attached to a substrate undergoing strain. An optical fiber containing quasi-sinusoidal microbends provides a mechanism for establishing a relationship between a diminished amplitude of the microbends and an increased optical transmission of the fiber. The mechanical wavelength (period) and amplitude of the microbends of a plastic optical fiber are optimized to measure strain. The mechanism used to stretch the optical fiber is mechanically expansive and designed for thermal insensitivity which exhibits relative displacement with respect to said first end of the microbend section as a function of temperature.
An article by N. Lagakos, et. al., "Microbend Fiber-optic Sensor," Applied Optics, Vol. 26, No. 1 1, pp. 2179, 1 June 1987, discusses a microbend fiber-optic sensor which is capable of sensing temperature. However, similar to the patents discussed above, the sensor utilizes a structure to impress non-existent microbends in the optical fiber. Additionally, the sensor does not employ an undulating tube as a confining structure to sense temperature.
A microbend fiber-optic pressure sensor is discussed by Fields et. al., J. Acoust. Soc. Am., Vol. 67, No. 3, Mar. 1980; the device does not measure temperature. The Fields structure uses one fiber-optic path and measures light intensity instead of optical phase shits, but requires a large, bulky, and complicated ridged pressure plate apparatus. Fields' device includes two mating ridged plates placed around a multi-mode step-index silica fiber, one end of which is illuminated by a laser and the other end monitored with a calibrated photometer. Motion is perpendicular to the axis of the fiber. A load applied to the pressure plates causes a quasi-sinusoidal distortion of the fiber. The device provides mechanically-induced amplification of light attenuation in the fiber caused by bending forces acting on the fiber; however, the size and bulkiness of the structure is simply not appropriate for a temperature sensor.
An article by J. N. Fields, "Attenuation of a Parabolic-Index Fiber with Periodic Bends," Applied Physics Letters (10), pp. 799-801, 15 May 1990, discusses measurements of excess loss induced by periodic distortions of multimode optical fibers having parabolic- or step-index profiles.
Cost and complexity have precluded the commercialization of many ingenious fiber-optic sensors. Thus, there is an existing need for a simple fiber-optic temperature sensor, which possesses the mechanical amplification of the Fields' structure without bulky, complicated, and ridged apparatus, and which measures the intensity of the light received rather than the interferometric phase shift.
The present invention differs from the above prior art in that it is explicitly a temperature sensor, and the optical fiber is deliberately configured in a manner to be strongly affected by temperature whereas the prior art teaches away from the effects of temperature. The prior art is concerned with eliminating the effect of temperature, such as degradation on an optical fiber so as to avoid fracturing it. Additionally, optimization of the mechanical wavelength and amplitude of the microbends is different for a step- or graded-index optical fiber to measure temperature. These parameters must be specifically selected for optimum transmission of the optical fiber over a wide range of temperature.